1. Field of the Invention
The present invention relates to a nuclear magnetic resonance imaging, and more particularly, to a nuclear magnetic resonance imaging suitable for imaging physiological function information on an interior of a body to be examined at high speed and high precision.
2. Description of the Background Art
As well known, the nuclear magnetic resonance imaging is a method for imaging microscopic chemical and physical information of matters by utilizing the nuclear magnetic resonance phenomenon in which the energy of a radio frequency magnetic field rotating at a specific frequency can be resonantly absorbed by a group of nuclear spins having unique magnetic moments which are placed in a homogeneous static magnetic field.
In this nuclear magnetic resonance imaging, images can be obtained in various contrasts such as an image in contrast emphasizing a longitudinal relaxation time T.sub.1 of nuclear spins (T.sub.1 image), an image in contrast emphasizing a transverse relaxation time T.sub.2 of nuclear spins (T.sub.2 image), an image in contrast emphasizing a density distribution of nuclear spins (density image), and an image in contrast emphasizing a parameter T.sub.2 * (T.sub.2 * image) which reflects both the transverse relaxation time T.sub.2 and a sudden phase change of nuclear spins due to a microscopic magnetic field inhomogeneity within a voxel.
On the other hand, as described in S. Ogawa et al.: "Oxygenation-Sensitive Contrast in Magnetic Resonance Image of Rodent Brain at High Magnetic Fields", Magnetic Resonance in Medicine 14, pp. 68-78, 1990, it is known that, among the hemoglobin contained in blood of a living body, the oxyhemoglobin contained in abundance in the arterial blood is diamagnetic, while the deoxyhemoglobin mainly contained in the venous blood is paramagnetic. Then, as described in R. M. Weisskoff et al.: "MRI Susceptometry: Image-Based Measurement of Absolute Susceptibility of MR contrast Agents and Human Blood", Magnetic Resonance in Medicine 24, pp. 375-383, 1992, it is also known that the diamagnetic oxyhemoglobin does not disturb a local magnetic field very much (magnetic susceptibility difference of 0.02 ppm with respect to living body tissues), but the paramagnetic deoxyhemoglobin has sufficiently large magnetic susceptibility difference with respect to surrounding tissues (magnetic susceptibility difference of 0.15 ppm with respect to living body tissues) to disturb the magnetic field so that the parameter T.sub.2 * is going to be shortened.
Also, as described in J. A. Detre, et al.: "Perfusion Imaging", Magnetic Resonance in Medicine 23, pp. 37-45, 1992, in some imaging schemes of the nuclear magnetic resonance imaging, when an amount or a speed of a local blood flow within living body tissues changes, the relaxation time (such as T.sub.1) of a living body seemingly appears to have changed, and an image contrast can be changed.
By utilizing the above noted properties, it is possible to image a change of blood flow or a change of oxygen density in blood due to a physiological function such as a cell activity within living body tissues including an activation of a visual area in a brain cortex caused by a light stimulation, as described for example in K. K. Kwong et al.: "Dynamic magnetic resonance imaging of human brain activity during primary sensory stimulation", Proc. Natl. Acad. Sci. U.S.A., Vol. 89. pp. 5675-5679, June 1992. Conventionally, an imaging scheme used in this type of imaging has been the echo planar scheme or the gradient echo scheme.
However, in these imaging schemes, a signal change (image contrast change) caused by a physiological function within a living body is quite minute. For this reason, conventionally, this minute signal change has been detected by calculating a difference of images before and after an occurrence of a physiological function phenomenon, or by applying a statistical processing. An example of a statistical data processing scheme to be used for this purpose is a scheme using the paired t-test as described in R. T. Constable, et al.: "Functional Brain Imagings at 1.5 T using Conventional Gradient Echo MR Imaging Techniques", Magnetic Resonance Imaging, Vol. 11, pp. 451-459, 1993. In the former case of using a difference, there is a need to obtain an image with high S/N ratio, while in the latter case of using a statistical processing, there is a need to obtain a plurality of images, so that the imaging time tends to be longer and it becomes easier for the images to be affected by a motion of the living body.
Moreover, it is also well known that the image distortion can be caused in nuclear magnetic resonance imaging when the static magnetic field distribution is inhomogeneous, and this image distortion becomes particularly noticeable in the imaging scheme for the T.sub.2 * image which is used in detecting a physiological function phenomenon such as a cell activity in a living body. In this regard, a method for correcting this image distortion by using processing such as the affine transformation is described in Japanese Patent Application No. 5-22759 (1993).
Thus, a detection of a minute signal change (image contrast change) caused by a physiological function within a living body requires a high S/N ratio image or a number of images, but for this reason the imaging time becomes long, and it becomes easier to receive an influence of a motion of a living body, such that it has been difficult to detect a minute signal change (image contrast change) caused by a physiological function within a living body. In fact, it is well known that a position and a size of a brain can change in synchronization with the heart beat, as described in B. P. Poncelet, et al.: "Brain Parenchyma Motion: Measurement with Cine Echo-Planar MR imaging", Radiology, Vol. 185, pp. 645-651, December 1992.
Thus, because of an influence of a body movement due to breathing or a heart beat, it has conventionally been impossible to accurately detect a signal change (image contrast change) caused by a physiological function such as a cell activity in a living body.
On the other hand, it has been discovered that, when a brain is imaged by the nuclear magnetic resonance imaging while giving a stimulation such as a visual stimulation, an image contrast changed portion due to presence/absence of the stimulation coincides with a physiologically known portion which reacts to the stimulation, that is, it is possible to image an active portion of a brain by the nuclear magnetic resonance imaging. The reason why it is possible to detect an active portion of a brain is considered as that the active portion requires more energy so that an amount of blood flowing into a region of the active portion and an amount of the deoxyhemoglobin in a vicinity of this region at a level of capillary vessels which play a role in an energy exchange are increasing. This changes in a state of bloods is called the BOLD (Blood Oxygen Level Dependent) effect, and it can be detected by the T.sub.2 * emphasizing pulse sequence which is sensitive to a change of the magnetic susceptibility, such as the EPI (Echo Planar Imaging), and the FE (Field Echo) with a long TE time.
A brain function image is an image of a brain in which an amount of change in the image contrast due to presence/absence of a stimulation is extracted as an active portion by means of a subtraction processing or a statistical processing on images taken with/without the stimulation by the nuclear magnetic resonance imaging. Thus, by using nuclear magnetic resonance imaging, it becomes possible to image an active portion of a brain at very high spatial resolution compared with other available means such as a SQUID used for a magnetoencephalography, and it provides a new scheme for detecting an active state of a brain. This brain function imaging scheme is attracting much attention because it is low invasive due to its use of bloods as a natural contrast medium, and it is readily realizable by using a widely spread nuclear magnetic resonance imaging apparatus.
Unlike the electro-physiological measurement scheme such as magnetoencephalography using a SQUID which has a good time resolution, this brain function imaging scheme is not suitable for the measurement of a latency with respect to a stimulation because of a presence of a delay time on an order of seconds for the activation after the stimulation as the active portion in the brain function image depends on a change in a state of blood flow. Also, although dependent on an amount of stimulation, an amount of change in the image contrast is about 0.5 to 5% of the image contrast which is rather small. Consequently, the active portion is detected in this brain function imaging scheme as a difference image between an image with a stimulation and an image without a stimulation. Here, in order to prevent a slight displacement between the images due to an influence of motion such as pulsation and to improve the S/N ratio, many images are taken under repeated presence/absence of a time series type stimulation, and the active portion is extracted by an averaging processing or a statistical processing.
Thus, the brain function imaging scheme is capable of imaging an active portion related to an activity in a brain rather than a physical shape information on a brain. However, in order to obtain a brain function image, it is necessary to image a brain repeatedly with and without a stimulation to a brain, so that the imaging time becomes long.
In addition, in order to be able to utilize a brain function image clinically, it is necessary to obtain a physical shape image and a blood vessel image strongly related to a brain function image at the same time and check a correlation relationship among these three images, but these three images are taken independently one by one, so that the entire imaging time becomes long, and the processing among the images becomes difficult because of a position displacement among the images which are taken independently.